(1) Field of the Invention
This invention relates to an enzyme sensor and, more particularly, to an enzyme sensor for measuring a substrate concentration by potentiometric or amperometric response on the basis of a molecular identifying function of an enzyme.
(2) Description of the Prior Art
Glucose sensors and urea sensors are known as examples of enzyme sensors utilizing an enzyme reaction. Glucose sensors utilize the following enzyme reaction: EQU glucose+O.sub.2 +H.sub.2 O.fwdarw.gluconolactone acid+H.sub.2 O.sub.2 enzyme (glucose oxidase: GOD)
According to the principle of the conventional glucose sensor, the concentration of the substrate glucose is measured by measuring the rate of in vivo O.sub.2 consumption or the rate of in vivo H.sub.2 O.sub.2 production using an enzyme sensor. In an enzyme sensor of this kind, however, an enzyme membrane is bonded on a gas-permeable membrane of an oxygen sensor or the like, so that the size of the enzyme sensor depends upon the oxygen sensor. Miniaturizing the enzyme sensor is difficult.
A method is available for measuring glucose concentration by using a pH sensor to detect the amount of change of a hydrogen ion concentration due to the production of gluconic acid, and use of a glass electrode pH sensor and an ion-selective FET sensor has been reported.
When a glass electrode is employed, however, response resulting from deposition of an enzyme layer is very unstable since the resistance of the electrode membrane is very high. Another drawback is the large size of the sensor. Though a reduction in size is possible if the ion-selective FET sensor is used, the adhesion of the enzyme layer presents a problem. Moreover, since a large-scale manufacturing apparatus is required, fabrication is not easy. The same problems are encountered in other enzyme sensors as well.